1. Field of the Invention
The present invention relates to a positioning control method and a positioning control apparatus which suppress mechanical vibration resulting from the movement, stop, etc., of a workpiece or a tool and exercise smooth relative position control of the workpiece and tool on a general machine tool, such as a turret punch press, or a machine tool such as a laser cutting machine, an electrical discharge machine or a wire bonding apparatus.
2. Description of the Background Art
There are conventional devices that serve as a positioning control apparatus for controlling, for example, a workpiece in a general machine tool, such as a turret punch press, or a machine tool such as a laser cutting machine, an electrical discharge machine or a wire bonding apparatus (hereinafter referred to as the machine tool), which suppresses the vibration of a movable part by detecting the acceleration of the movable part and feeding the result of detection back to a control section. FIG. 24 shows an apparatus disclosed in Japanese Laid-Open Patent Publication No. 136582 of 1989. In FIG. 24, the numeral 1 indicates a servo amplifier, 2 denotes a driving apparatus, e.g., a motor, 3 represents transmission means consisting of a joint and a lead screw, 4 designates a movable part, such as a traveling table, which is moved by the transmission means 3, 5 indicates a tacho-generator for detecting the velocity VD of the driving apparatus 2, 6 shows acceleration detector means for detecting the acceleration .alpha.L of the movable part 4, 7 designates an integrator for integrating the acceleration .alpha.L detected by the acceleration detector means 6 and providing the velocity VL of the movable part 4, 8 represents an amplifier for amplifying a difference VL-VD between the velocity VL of the movable part 4 and the velocity VD of the driving apparatus 2, and 9 indicates a filter.
The difference VL-VD between velocity VL provided by integrating the acceleration .alpha.L of the movable part 4 and the velocity VD of the driving apparatus 2 is the extraction of movable part 4 vibration attributable to mechanical natural oscillation. The vibration of the movable part 4 is suppressed by feeding back the signal of that difference with polarity as indicated in the drawing through the amplifier 8 of appropriate gain and the filter 9 of proper time constant.
In the meantime, the machine tool is designed to move movable parts, e.g., a machining table and a tool member in the directions of two or more axes, i.e., an X-axis direction, a Z-axis direction (and/or a Y-axis direction). Since a workpiece is machined by the relative motion of the workpiece and tool member fixed on their corresponding movable parts of the machine tool, it is important to suppress the detrimental relative vibration of the workpiece and tool member in improving machining accuracy. This vibration problem will now be described with reference to the drawings.
FIG. 25 is a simplified diagrammatic view of a machine tool, wherein 4 indicates a machining table acting as a first movable part which can be moved in the X-axis direction, 2 designates a first driving apparatus for driving the machining table 4 in the X-axis direction, 5 denotes position detector means for detecting, for example, the angular position of the first driving apparatus 2, 3 indicates first transmission means consisting of a joint and a lead screw which transmit the driving force of the first driving apparatus 2, 260 shows a bed on which the machining table 4, first driving apparatus 2, etc. are mounted, 261 represents a machining head serving as a second movable part which can be moved in the Z-axis direction, 262 shows a second driving apparatus for driving the machining head 261, 263 indicates second transmission means consisting of a joint and a lead screw which transmit driving force to the machining head 261, and 264 denotes a column on which the machining head 261 and second driving apparatus 262 are mounted.
The conventional apparatus constructed as described above machines a workpiece to a desired shape while simultaneously controlling the positions of the machining table 4 secured with the workpiece in the X-axis direction and the machining head 261 loaded with a tool in the Z-axis direction by means of the first and second driving apparatuses 2, 262 via the first and second transmission means 3, 263, respectively. In the process of machining this workpiece, when the machining table 4 is accelerated or decelerated in the X-axis direction, the machining table 4 vibrates in the X-axis direction since the first transmission means 3 is made of an elastic material. Further, the bed 260 vibrates due to the reaction to the acceleration or deceleration of the machining table 4, whereby deflective vibration in the X-axis direction takes place in the column 264.
FIG. 26(a) is a waveform diagram which illustrates the acceleration .beta.X of the machining head 261, the acceleration .alpha.X of the machining table 4, and the relative acceleration .alpha.X-.beta.X of the two when the machining table 4 at a stop is moved in the +X-axis direction on a conventional apparatus for which measures against vibration have not been taken. As shown in FIGS. 26(a) and 26(c), when the machining table 1 is started without vibration measures being taken, the relative acceleration waveform of the machining table 4 and machining head 261 contains more detrimental residual vibration as compared to the acceleration waveform of the machining table 4. Consequently, the machined surface of the workpiece is undulated due to the X-axis direction relative vibration of the machining table 4 and machining head 261, thereby degrading machining accuracy. Therefore, the vibration problem caused by the relative vibration of the machining table 4 and machining head 261 could not be solved satisfactorily by merely detecting the acceleration of only one axis, e.g., the machining table 4, and making feedback compensation as in the conventional apparatus shown in FIG. 24.
It is, accordingly, an object of the present invention to overcome the disadvantages in the conventional apparatus by providing a positioning control method which suppresses relative vibration and achieves high-speed, accurate positioning control and an apparatus which carries out that method.